Optical lithographical imaging system including optical transmission diffraction devices

ABSTRACT

In an optical lithographical system (e.g., 100) for printing features of a patterned mask (e.g., 103) into a workpiece (e.g., 120), such as in a semiconductor device, a one- or two-dimensional (depending on the features of the mask) optical transmission phase-shift diffracting device (e.g., 107) is inserted between an optical condensor (e.g., 105) and the patterned mask whose features are to be printed into the workpiece. The diffracting device is designed so as to enable an imaging lens system (e.g., 102) to collect more than merely a single order of the resulting diffraction patterns of the features of the mask and, by varying the spatial periodicity of the diffracting device, to tailor the illumination incident on the mask in accordance with the features (e.g., 114, 115) of the mask.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part application of E. L. Raab etal. 1-21-33, Ser. No. 07/999064, filed Dec. 31, 1992, now abandoned.

TECHNICAL FIELD

This invention relates to optical lithography and more particularly tosuch systems that include (patterned) lithographic masks. The masks arealso called "reticles".

BACKGROUND OF THE INVENTION

An optical lithographic fabrication system is used in prior art fordelineating features in a workpiece. Typically the workpiece comprises asemiconductor wafer (substrate), together with one or more layers ofmaterial(s) (not shown) located on a top major surface of the wafer.

During operation of the optical lithographical system, typicallymonochromatic optical radiation of wavelength λ is emitted by an opticalsource, such as a mercury lamp. This radiation propagates successivelythrough an aperture in an opaque screen, an optical condensor such as anoptical condensing lens (or condensing lens system), a lithographic maskor reticle, and an optical imaging lens or imaging lens system. Theoptical radiation emanating from the reticle is focused by the imaginglens onto a photoresist layer located on the top major surface of theworkpiece. Thus, the pattern of the mask--that is, the pattern of itstransparent and opaque portions--is imaged on the photoresist layer.

The mask or reticle illustratively includes a uniformly thick, opticallytransparent substrate, typically made of glass. On the underside (bottomsurface) of this transparent substrate is located a patterned opaquelayer, typically made of chromium, having two illustrative portions: (1)an illustrative portion which is suitable for forming an imagecomprising an isolated circular or an isolated square bright spot(localized area) on the photoresist layer, and (2) another illustrativeportion which is composed of mutually parallel opaque stripes suitablefor forming an image comprising mutually parallel lines and spaces onthe photoresist layer. The illustrative portion which is suitable forforming the isolated bright spots typically is composed of an opaquesolid layer having an isolated circular or isolated square aperture init.

Depending upon whether the photoresist layer comprises negative orpositive resist material, when it is subjected to a development process,typically a wet developer, the material of the photoresist respectivelyremains or is removed at, and only at, areas on which the opticalradiation was incident. Thus, the pattern of the mask is transferred tothe photoresist layer, whereby a patterned photoresist layer is formed.

Subsequent etching process, such as wet etching or dry plasma etching,can then remove portions of the workpiece in accordance with the patternof the photoresist layer and hence in accordance with the pattern of themask. That is to say, portions of the workpiece are removed from the topsurface of the workpiece at areas underlying those regions where thematerial of the photoresist layer was removed by the development processbut not at areas underlying those regions where the photoresist remains.Alternatively, instead of etching the workpiece, impurity ions can beimplanted into the workpiece at areas underlying those regions where thephotoresist was removed by the development process but not at areasunderlying those regions where the photoresist remains. In accordancewith yet another alternative, such as a "lift-off" process,metallization can be deposited or otherwise formed on the workpiece onlyat areas underlying those regions where the photoresist was removed.Thus, in any event, the pattern of features of the mask--i.e., eachfeature of the mask such as the above-mentioned parallel lines andspaces as well as the isolated aperture--ultimately is transferred tothe workpiece. Such pattern transfer is desired, for example, in the artof semiconductor integrated circuit fabrication.

In fabricating such circuits, it is desirable, for example, to have asmany transistors per wafer as possible. Hence, it is desirable to haveas small a transistor or other feature size as possible, such as thefeature size of a metallization stripe--i.e., its transverse width--orof an aperture in an insulating layer which is to be filled with metal,for example, in order to form electrical connections between one levelof metallization and another or to form metallization lines.

When the size of a feature on the mask is made to be so small that theimaging lens system collects the zero'th order of the diffractionpattern of the feature but collects relatively little of the first orderof the diffraction pattern of the feature, then a problem arises in thatthe edges, including corners, of the image of the feature on thephotoresist become indistinct, and therefore very small featuresundesirably are not printed. Hence the optical contrast of the featureas focused as an optical image on the photoresist layer and transferredto the workpiece deteriorates. As is well known, this lower opticalcontrast results in a poorer resolution of the feature by thephotoresist layer. In practice, this problem typically arises when thesize of the feature on the mask is smaller than approximately(0.7)λ/(NA), where NA is the numerical aperture on the mask side("object side") of the imaging lens. Typically this NA is approximatelyequal to 0.1 in case the magnification m is in the approximate range of0.20 to 0.25. Also, the depth of focus of features of the mask as imagedon the photoresist layer is often limited to values than can be lowerthan are desirable from the standpoint of desired resolution.

SUMMARY OF THE INVENTION

In order to improve the contrast and hence the resolution, as well as toincrease the depth of focus, a one-dimensional or a two-dimensionaloptical transmission diffraction device is inserted between the opticalcondensor and the patterned mask in the above-described opticallithographical system. The diffracting device is, in one embodiment, atransmission diffracting device having a set of relatively thicktransparent regions with a set of boundaries running parallel to onedirection on a transparent substrate. By "transparent" is meantoptically transparent with respect to radiation of the wavelength λ tobe used in the lithographic system. Alternatively, the device has firstand second (or more) sets of transparent regions, each of the regions inthe first set having boundaries running parallel to a first spatialdirection, and each of the regions in the second set having boundariesrunning parallel to a second spatial direction that is not parallel tothe first direction. In another alternative embodiment theone-dimensional optical diffracting device advantageously can be, atleast in a localized area thereof that is to overlie mutually parallelfeatures (lines and spaces) in the pattern of the mask, an array ofparallel alternating opaque and transparent stripes (i.e., aone-dimensional transmission grating). Moreover, two-dimensionaldiffracting device can be, at least in a localized area thereof, across-grating (i.e., one set of parallel transparent stripes oriented ata prescribed angle with respect to another set of parallel transparentstripes located on an opaque coating on a transparent substrate). It isalso advantageous that the diffracting device be designed such that theimaging lens system collects not only the zero'th order of each of thediffraction patterns of each of the features of the mask but also eitherthe +1 or the -1 order of each of these diffraction patterns. It is alsoadvantageous that all the thick regions have thicknesses that areuniform and are mutually equal, each of these thick regions typicallysupplying a phase shift equivalent nπ radian, where n is an odd integer.

It is also advantageous, in a specific embodiment, that at least some ofthe thick transparent regions form an array of concentric circularrings. It is also advantageous in a specific embodiment that the twonon-parallel spatial directions are mutually perpendicular and that anarea of the diffracting device form a checkerboard pattern of thickregions. It is also advantageous in another specific embodiment that thethick regions form at least first and second sets of elongated stripes,all the stripes in the first set running perpendicular to all thestripes in the second set. It is also advantageous in another specificembodiment that the thick regions form at least two sets of elongatedstripes running in at least two non-parallel spatial directions,respectively. It is also advantageous in another specific embodimentthat the thick regions form at least three sets of elongated stripesrunning in three mutually non-parallel spatial directions, respectively.

It is also advantageous in another specific embodiment that thediffracting device is divided into a pattern of squares or rectangleseach of which contains a multiplicity of thick regions in the form ofstripes running along but one of a plurality, preferably more than two,of different possible spatial directions.

It is also advantageous in yet another specific embodiment that thediffracting device is divided into different areas in which the sizes ofsquares in the checkerboard pattern, or the distances betweenneighboring elongated stripes, are different in different areas. It isalso advantageous in yet another specific embodiment, optionally inaddition to the sizes of the square being different in different areas,that each square contains a fine structure of thick regions in the formof elongated parallel stripes, and that in any square these stripes allrun in one of at least two or more different directions.

It is further advantageous that the spatial periodicity P of thediffracting device varies from (localized) area to (localized) area ofthe diffracting device in an increasing or decreasing manner,respectively, in accordance the respective increasing or decreasingsizes of features located on the patterned mask in the immediateneighborhood of the respective areas of the diffracting device. It isalso advantageous that, in addition to the spatial periodicity P varyingfrom localized area to localized area of the diffracting device, theorientation and shapes of the thick regions of the diffracting deviceshould vary in such a manner from area to area so as to optimize theresolution of the features located in the respective neighborhoods ofthe respective areas of the mask. It is also advantageous that thediffracting device is a structure that is physically separate anddistinct from the patterned mask: for example, the diffracting device isa transparent substrate formed by transparent parallel slab on one ofwhose major surface the thick regions have been deposited, this slabbeing a separate body from another transparent slab that forms asubstrate of the patterned mask.

This invention also involves a photolithographic method of printing(i.e., transferring) a pattern of features into a workpiece, inaccordance with a pattern of features of a patterned mask, comprisingthe steps of

(a) directing optical radiation onto the condensor, through thediffraction device, and through the mask, while focusing the opticalradiation emanating from the mask onto a photoresist layer located on aworkpiece, whereby an optical image of the mask is formed on thephotoresist layer;

(b) developing the photoresist layer, whereby a feature is formedtherein in accordance with the image of the mask; and

(c) transferring into the workpiece the feature in the photoresistlayer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an optical lithographic system, partly in cross section, inaccordance with the invention;

FIG. 2 is a bottom view diagram of a one-dimensional diffracting devicefor use in the optical system shown in FIG. 1, in accordance with yetanother specific embodiment of the invention;

FIG. 3 is a bottom view diagram of a two-dimensional diffracting devicefor use in the optical system shown in FIG. 1, in accordance with aspecific embodiment of the invention;

FIG. 4 is a bottom view diagram of a two-dimensional diffracting devicefor use in the optical system shown in FIG. 1, in accordance withanother specific embodiment of the invention; and

FIG. 5 is a bottom view diagram of a two-dimensional diffracting devicefor use in the optical system shown in FIG. 1, in accordance withanother specific embodiment of the invention.

Only for the sake of clarity, none of the drawings is to scale. Theshaded areas in FIGS. 2, 3, 4, and 5 represent relatively thick regionsof the respective diffracting devices, typically supplying phase shiftsequivalent to nπ radian, where n is an odd integer.

DETAILED DESCRIPTION

Referring now to the drawing, FIG. 1 shows a typical opticallithographic fabrication system 100 suitable for delineating (printing)features in a workpiece 120 such as a semiconductor wafer.

During operation of the system 100 typically monochromatic opticalradiation of wavelength λ is emitted by an optical source 106, such as amercury lamp. This radiation propagates successively through an aperturein an opaque screen 105, an optical condensor such as an opticalcondensing lens (or condensing lens system) 104, a lithographic mask orreticle 103, and an optical imaging lens or lens system 102. The opticalradiation emanating from the reticle 103 is focused by the lens 102 ontoa photoresist layer 101 located on the top major surface of theworkpiece 120. Thus, the pattern of the mask 103--that is, the patternof its transparent and opaque portions--is imaged on the photoresistlayer 101.

The mask or reticle 103 illustratively includes a uniformly thicktransparent substrate 113, typically made of glass. On the underside(bottom surface) of this transparent substrate 103 is located apatterned opaque layer, typically made of chromium, having twoillustrative portions: (1) a portion 114 which is suitable for formingan image comprising an isolated circular or an isolated square brightspot on the photoresist layer 101, and (2) another portion which iscomposed of mutually parallel elongated opaque stripes 115 suitable forforming an image comprising mutually parallel lines and spaces on thephotoresist layer 101. The portion 114 typically is composed of anopaque solid layer having an isolated circular or an isolated squareaperture in it.

Depending upon whether the photoresist layer 101 comprises negative orpositive resist material, when it is subjected to a development process,typically a wet developer, the material of the photoresist respectivelyremains or is removed at, and only at, areas on which the opticalradiation was incident. Thus, the pattern of the mask is transferred tothe photoresist layer 101, whereby a patterned photoresist layer isformed.

Subsequent etching processes, such as wet etching or dry plasma etching,can then remove portions of the workpiece 120 in accordance with thepattern of the photoresist layer 101 and hence in accordance with thepattern of the mask 103. That is to say, portions of the workpiece 120are removed from the top surface of the workpiece 120 at areasunderlying those regions where the material of the photoresist layer 101was removed by the development process but not as areas underlying thoseregions where the photoresist remains. Alternatively, instead of etchingthe workpiece, impurity ions can be implemented into the workpiece 120at areas underlying those regions where the photoresist was removed bythe development process but not at areas underlying those regions wherethe photoresist remains. In accordance with yet another alternative,such as a "lift-off" process, metallization can be deposited orotherwise formed on the workpiece 120 only at areas underlying thoseregions where the photoresist was removed. Thus, in any event, thepattern of features of the mask 103--i.e., each feature of the mask suchas the above-mentioned parallel lines and spaces as well as the isolatedaperture--is transferred to the workpiece 120. Such pattern transfer isdesired, for example, in the art of semiconductor integrated circuitfabrication.

In order to improve optical contrast and hence resolution, as well as toincrease the depth of the focus, an optical transmission diffractingdevice 107 is located between the condensor 104 and the mask 103. Thisdiffracting device 107 is designed so as to enable the imaging lens 102to collect more than merely the zero'th order diffraction pattern ofeach of the features of the mask 113, advantageously to collect theorders of the diffraction pattern of each feature of the mask 113corresponding to its zero'th and either its +1 or its -1 orderssimultaneously. Moreover, the features of the diffracting device 107advantageously are made to run locally at predetermined angles withrespect to the features of the mask 103. For example, a feature in thediffracting device 107 typically is formed by a boundary of a uniformlythick elongated transparent stripe 118 deposited on a uniformly thicktransparent substrate 117. Alternatively, the substrate 117 can be aninitially parallel transparent slab that has been spatially selectivelyetched to produce the desired diffracting device 107. Alternatively, thesubstrate 117 can be an initially parallel transparent slab that hasbeen spatially selectively etched to produce the desired diffractingdevice 107. In a checkerboard diffracting pattern, such as the device200 (FIG. 3), for purposes of measuring its periodicity P, the featuresof the diffracting pattern run at an angle of +π/4 and -π/4 radian withrespect to boundaries of the thick regions, i.e., with respect to thesides of the squares. Specific embodiments 200, 300, 400, and 500 of thediffracting device 107 are shown in FIGS. 2, 3, 4, and 5, respectively.

For example, FIG. 2 shows a one-dimensional optical transmissiondiffraction grating device 200 suitable for use as the diffractingdevice 107 in the optical lithographical system 100 (FIG. 1). Thisdevice 200 comprises an optically transparent substrate slab, typicallyof quartz, having mutually parallel thick elongated transparent regions201. Each of these regions 201 has a uniform thickness that is equal tothe thicknesses of the others. As used herein, the term "transparent"denotes optically transparent to the optical radiation of wavelength λto be used in the system 100. Typically, the thick regions 201 areformed by chemical vapor deposition of a uniformly thick layer ofsilicon dioxide followed by lithographic masking and etching of thislayer of silicon dioxide.

The thickness of the thus formed relatively thick regions 201advantageously is selected to be equal to the equivalent of aphase-shift of nπ radian with respect to the radiation of wavelength λ,where n is an odd integer preferably equal to one. The distance ofseparation between the bottom surface of these thick regions 201 and thetop surface of the substrate 113 of the mask 103 is typically in theapproximate range of between 0.2 cm and 2.0 cm for the sake of uniformillumination of the mask 103.

Instead of the elongated parallel regions 201 being thick andtransparent, they can be opaque or other type of parallel grating lines,for example can be formed by means of e-beam writing on the same kind ofchromium-covered glass plates used for ordinary mask making, e.g., formaking the mask 113.

The grating device 200 is especially useful in conjunction with parallellines and spaces as features of the mask 103. Advantageously the spatialdirections, along which the elongated regions 201 run, vary from area toarea on the device 200 so as to run at least approximately parallel tothe line and space features of the mask 103 that are located in therespective neighborhoods of the respective areas of the device 200.

The spatial periodicity P of the diffracting device 200 advantageouslycan be made to vary from area to area on this device 200 in anincreasing or decreasing manner in accordance with the respectiveincreasing or decreasing sizes of the (line and space) features of themask 103 that are located in the respective neighborhoods of thoserespective areas. In this way, the illumination incident on the mask 103can be tailored specifically to individual areas of this mask 103 andhence of the photoresist layer 101.

For UV lithography, the widths of the thick regions 201 (or the widthsof opaque grating lines) are advantageously in the approximate range of1.25-to-2.5 microns, as are the distances between neighboring suchregions (or the distances being adjacent grating lines); for deep UVlithography, the approximate range is advantageously 0.8-to-2.0 microns.

Referring again to the drawing, FIG. 3 shows a two-dimensional opticaltransmission diffracting device 300 suitable for use in the opticallithographical system 100 (FIG. 1) as the diffracting device 107. Thisdevice 300 is composed of an optically transparent substrate slab,typically of quartz, with thick optically transparent regions 301, eachof these regions 301 having a uniform thickness in common with theothers. Typically, the thick regions 301 are formed by chemical vapordeposition of a uniformly thick layer of silicon dioxide followed bylithographic masking and etching of this layer of silicon dioxide.

The thickness of the thus formed relatively thick regions 301advantageously is selected to be equal to the equivalent of aphase-shift of nπ radian with respect to the radiation of wavelength λ,where n is an odd integer preferably equal to one. Also, the relativelythick regions 301 thus all have a common thickness. The distance ofseparation between the bottom surface of these thick regions 301 and thetop surface of the substrate 113 of the mask 103 is typically in theapproximate range of between 0.2 cm and 2.0 cm for the sake of uniformillumination of the mask 103.

In FIG. 3 the diffracting pattern formed by the thick regions 301 can bedivided into two types of areas: one area consisting of a checkerboardpattern of squares that is oriented parallel to the X and Y axes, andanother area consisting of a checkerboard pattern of squares that isoriented at an angle of π/4 radian with respect to the X axis.Advantageously, the respective extents of these areas are selected suchthat (1) the area whose pattern of squares has boundaries that areoriented parallel to the X axis is placed in the neighborhood offeatures in the mask 103 that run at an angle of approximately π/4radian with respect to the X or Y axis, and (2) the area whose patternof squares has boundaries that are oriented at an angle of π/4 radianwith respect to the X axis is placed in the neighborhood of features inthe mask 103 that runs approximately parallel to the X or Y axis. Itshould be understood, however, that--depending on the (majority)orientation of the features (lines and spaces, apertures, etc.) of thepattern of the mask 103 in the neighborhood of the area underconsideration and on the degree of improvement that is desired in theresolution of the image to be formed on the photoresist layer 101--onlyone type of these areas is essential; the other can be omitted.Typically, the (equal) lengths of the sides of each of the squares is inthe approximate range of between (0.7)λ/(NA) and (3.5)λ/(NA) preferablyin the approximate range of between (0.8)λ/(NA) and (1.7)λ/(NA), whereagain NA is the numerical aperture on the mask side of the imaging lens102. Typically, this numerical aperture (NA) is approximately equal to0.1.

Instead of the squares (FIG. 3), rectangles can be used.

In FIG. 3 also, the spatial periodicity P of the (checkerboard) patternof the diffracting device 300 is equal to the distance between thecenters of two squares that touch each other at, and only at, a mutualcorner; this distance in turn is equal to the length of the diagonal ofone of the squares. Moreover, advantageously the diffracting device 300is located in the system 100 (FIG. 1) such that the distance between thebottom surfaces of the thick regions 301 and the bottom surface of thetransparent substrate 113 of the mask 103 are all at least approximatelymutually equal and are all within the approximate range of between 100 Pand 1000 P.

The spatial periodicity P can be made to vary from area to area on thediffraction device 300 in an increasing or decreasing manner inaccordance with the respective increasing or decreasing sizes of thefeatures of the mask 103 located in the immediate neighborhoods of theserespective areas. In this way, the illumination striking the mask 103can be tailored specifically to individual areas of the mask 103 andhence of the photoresist layer 101.

FIG. 4 shows a two-dimensional optical transmission diffracting device400 suitable for use as the diffracting device 107 in the opticallithographical system 100 (FIG. 1), in accordance with anotherembodiment of the invention. This diffracting device 400 has relativelythick region segments 401 and, except for the hexagonal elongated stripeshape of these thick region segments 401, is similar in all (other)respects to the above-described diffraction device 300.

As indicated in FIG. 4, the pattern of the diffracting device 400 isformed by a two-dimensional array of the segments 401, each of which isa relatively thick region as compared with the remainder of the device400. Except for the central hexagon, the segments 401 run parallel tothree different directions that are oriented at angles that areapproximately equal to 0, π/3, and 2π/3 radian, respectively, withrespect to the X axis.

As further indicated in FIG. 4, a hexagonal ring is formed by each groupof a plurality of six connected segments 401. A plurality of concentricrings forms an overall hexagon, and a closely packed array of overallhexagons is formed by the overall hexagons. Although the drawing (FIG.4) indicates a plurality of only three rings per overall hexagon,advantageously the number of such rings per overall hexagon is in theapproximate range of between five and ten. Advantageously also, thewidths of all the six line segments in every ring are equal to oneanother and to the spaces between neighboring rings. The mutually equaldistances between centers of neighboring parallel segments 401 areadvantageously in the approximate range of between λ/(NA) and 5λ/(NA),where again (NA) is the numerical aperture on the mask side of theimaging lens 102 and is typically approximately equal to 0.1.

In FIG. 4, the spatial periodicity P of the resulting hexagonalclose-pack array of the diffracting device 400 is equal to the distancebetween centers of neighboring overall hexagons. Moreover,advantageously the diffracting device 400 is located in the system 100(FIG. 1) such that the distances between the bottom surfaces of thethick segments 301 and the bottom surface of the transparent substrate113 of the mask 103 are all at least approximately mutually equal andare all in the approximate range of between 100 P and 1000 P. Again, theperiodicity P can be made to vary, for the same reason as discussedabove in connection with FIG. 3.

Instead of the array (FIG. 4) of pluralities hexagonal rings,pluralities of concentric circular rings together with segments ofcircular rings can be used, an overall circle being formed by aplurality of such concentric circular rings, with the resulting overallcircles forming an array that is either hexagonally closely packed orcircularly closely packed. The segments of circular rings advantageouslyoccupy the otherwise resulting empty (non-diffracting) spaces located atareas on the diffracting device more remote from centers of therespective circular rings.

FIG. 5 shows a two-dimensional optical transmission diffracting device500 suitable for use as the diffracting device 107 (FIG. 1), inaccordance with yet another embodiment of the invention. Thisdiffraction device 500 has relatively thick elongated stripe regions 501and, except for the elongated stripe shape and orientations of thesethick regions 501, is similar in all other respects to theabove-described diffracting device 400. In particular, the diffractingdevice 500 has four different types of areas in each of which thestripes are oriented at approximately 0, π/4, π/2, or 3π/4 radian,respectively, with respect to the X axis. Typically, each of the areasis in the form of a square that contains a plurality of stripes:advantageously, for example, between five and ten stripes in each of theareas of the types in which the stripes all run at angles that areapproximately equal to 0 or π/2 radian with respect to the X axis, andbetween seven and fourteen stripes in each of the other types of areas.

Advantageously, the widths of thick stripe regions 501 are all mutuallyequal and are equal to the mutually equal spacings between neighboring(parallel) elongated stripe regions 501. Typically, the distance betweencenters of these neighboring stripe regions 501 is in the approximaterange of between π/(NA) and 5π/(NA), where again (NA) is the numericalaperture on the mask side of the imaging lens 102 and is typicallyapproximately equal to 0.1.

In FIG. 5, the spatial periodicity P of the device 500 is, for example,equal to the distance between the centers of two overall squares whosestripes 501 run at π/4 radian with respect to the X axis and that toucheach other at, and only at, a mutual corner; in turn this distance andhence this spatial periodicity P is equal to the length of the diagonalof one of these overall squares. Moreover, advantageously, thediffracting device 500 is located in the system 100 (FIG. 1) such thatthe distances between the bottom surface of the thick regions 501 andthe bottom surface of the transparent substrate 113 of the mask 103 areall approximately mutually equal and are all in the approximate range ofbetween 100 and 1000 P. Again, the periodicity P can vary as discussedabove in connection with FIG. 3. Also, in FIG. 5, instead of each squarebeing occupied by a plurality of parallel stripes, each square isoccupied by a checkerboard pattern of the kind shown in FIG. 3, thecheckerboard pattern within a given square being oriented eitherparallel to the X axis or at an angle thereto which is approximatelyequal to π/4 radian.

Although the invention has been described in detail in terms of specificembodiments, various modifications can be made without departing fromthe scope of the invention. For example, any of the diffracting devices200, 300, 400, and 500 can be oriented (i.e., can be "flipped") suchthat its substrate is located closer to the mask 103 than are its thickregions. Finally, the substrate of the diffracting device 200 can beomitted by forming the grating 201 on the top surface of the substrate200, and the substrates of the diffracting devices 300, 400, and 500 canlikewise be omitted while the thick regions 301, 401, or 501,respectively, can be directly deposited on the top surface of thetransparent substrate 113 of the mask 103--all at the expense ofprocessing yield of satisfactory resulting integrated mask devices, andprovided, however, that this substrate 113 can be fabricated with athickness that is compatible with the required distance of separationbetween the bottom surface of the substrate 113 and the grating 201 orbetween the bottom surface of the substrate 113 and the bottom surfaceof the regions 301, 401, or 501, respectively.

We claim:
 1. An optical lithographic system comprising: an opticaltransmission diffracting device located between an optical condensormeans and a pattern mask, the optical transmission diffraction gratingdevice being separated from the patterned mask by a distance in theapproximate range .Iadd.of .Iaddend.about 0.2 cm . .and.!. .Iadd.to.Iaddend.2.0 cm.
 2. The system of claim 1 in which the diffractingdevice comprises a one-dimensional diffracting device.
 3. The system ofclaim 1 in which the diffracting device comprises a two-dimensionaldiffracting device.
 4. An optical system in accordance with claim 3 inwhich the diffracting device had a set of relatively thick transparentregions whose boundaries run parallel to one another, and in which allthe thick regions have thicknesses that are uniform and are mutuallyequal.
 5. The system of claim 4 in which the diffracting device islocated on a substrate that is physically separate and distinct from asubstrate on which the pattern mask is located.
 6. The system of claim 4in which the thick regions form at least first and second sets ofelongated stripes, all the stripes in the first set runningperpendicular to all the stripes in the second set.
 7. An opticallithographic system in accordance with claim 1, in which the mask anddiffracting device are integrated on a single transparent parallel slabthat has first and second major surfaces, on the first major surface ofwhich is located a first pattern of opaque layers in order to form thepatterned mask, and on the second major surface of which is located asecond pattern of thin and thick transparent regions in order to formthe diffraction grating.
 8. The system of claim 7, the thick regionsforming at least first and second sets of elongated stripes, all thestripes in the first set running perpendicular to all the stripes in thesecond set.
 9. The system of claim 7 in which at least some of the thickregions form a plurality of concentric circular rings.
 10. As opticallithographical system in accordance with claim 4, in which the thickregions form at least first and second sets of elongated stripes runningin non-parallel first and second spatial directions, respectively. 11.An optical lithographical system in accordance with claim 4 in which thethick regions form at least first, second, and third sets of elongatedstripes running in mutually non-parallel first, second, and thirdspatial directions, respectively.
 12. A system in accordance with claim11 in which the diffracting device has a pattern of squares orrectangles in any one of which the stripes all run in one of the threeor more directions.
 13. An optical lithographical system in accordancewith claim 4 in which the thick regions form at least first, second,third, and fourth sets of elongated stripes running in mutuallynon-parallel first, second, third, and fourth spatial directions.
 14. Asystem in accordance with claim 13 in which at least a portion of thediffracting device is divided into squares or rectangles in any one ofwhich all the stripes are members of one, and only one, of the sets. 15.A system in accordance with claim 4 in which the thick regions form acheckerboard pattern of squares or rectangles.
 16. The system of claim15 in which the orientation of the squares or rectangles varies fromarea to area of the diffracting device.
 17. An optical lithographicalsystem in accordance with claim 4, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15,or 16, in which the mask has mask feature sizes that vary from area toarea of the mask, and in which the diffracting device has a spatialperiodicity that varies from area to area of the device in amonotonically increasing or decreasing manner, respectively, inaccordance with the respective increasing or decreasing mask featuresizes located in the neighborhoods of the respective areas of thediffracting device.
 18. A photolithographic method comprising the stepsof(a) directing optical radiation onto the system of claim 1, 4, 6 7, 8,9, 10, 11, 12, 13, 14, 15, or 16 while focusing the optical radiationemanating from the system onto a photoresist layer located on aworkpiece, whereby an optical image of the mask is formed on thephotoresist layer; (b) developing the photoresist layer, whereby afeature is formed therein in accordance with the image of the mask; and(c) transferring into the workpiece the feature in the photoresistlayer.